Electronic circuits require passive electronic components such as resistors, capacitors, and inductors. A recent trend is for passive electronic components to be embedded or integrated into the organic printed circuit board (PCB). The practice of embedding capacitors in printed circuit boards allows for reduced circuit size and improved circuit performance. Embedded capacitors, however, must meet high reliability requirements along with other requirements, such as high yield and performance. Meeting reliability requirements involves passing accelerated life tests. One such accelerated life test is exposure of the circuit containing the embedded capacitor to 1000 hours at 85% relative humidity, 85° C. under 5 volts bias. Any significant degradation of the insulation resistance would constitute failure.
High capacitance ceramic capacitors embedded in printed circuit boards are particularly useful for decoupling applications. High capacitance ceramic capacitors may be formed by “fired-on-foil” technology. Fired-on-foil capacitors may be formed from thick-film processes as disclosed in U.S. Pat. No. 6,317,023B1 to Felten or thin-film processes as disclosed in U.S. Patent Application 20050011857 A1 to Borland et. al.
Thick-film fired-on-foil ceramic capacitors are formed by depositing a thick-film capacitor dielectric material layer onto a metallic foil substrate, followed by depositing a top copper electrode material over the thick-film capacitor dielectric layer and a subsequent firing under copper thick-film firing conditions, such as 900-950° C. for a peak period of 10 minutes in a nitrogen atmosphere.
The capacitor dielectric material should have a high dielectric constant (K) after firing to allow for manufacture of small high capacitance capacitors suitable for decoupling. A high K thick-film capacitor dielectric is formed by mixing a high dielectric constant powder (the “functional phase”) with a glass powder and dispersing the mixture into a thick-film screen-printing vehicle.
During firing of the thick-film dielectric material, the glass component of the dielectric material softens and flows before the peak firing temperature is reached, coalesces, encapsulates the functional phase, and finally forms a monolithic ceramic/copper electrode film.
The foil containing the fired-on-foil capacitors is then laminated to a prepreg dielectric layer, capacitor component face down to form an inner layer and the metallic foil may be etched to form the foil electrodes of the capacitor and any associated circuitry. The inner layer containing the fired-on-foil capacitors may now be incorporated into a multilayer printed wiring board by conventional printing wiring board methods.
The fired ceramic capacitor layer may contain some porosity and, if subjected to bending forces due to poor handling, may sustain some microcracks. Such porosity and microcracks may allow moisture to penetrate the ceramic structure and when exposed to bias and temperature in accelerated life tests may result in low insulation resistance and failure.
In the printed circuit board manufacturing process, the foil containing the fired-on-foil capacitors may also be exposed to caustic stripping photoresist chemicals and a brown or black oxide treatment. This treatment is often used to improve the adhesion of copper foil to prepreg. It consists of multiple exposures of the copper foil to caustic and acid solutions at elevated temperatures. These chemicals may attack and partially dissolve the capacitor dielectric glass and dopants. Such damage often results in ionic surface deposits on the dielectric that results in low insulation resistance when the capacitor is exposed to humidity. Such degradation also compromises the accelerated life test of the capacitor.
It is also important that, once embedded, the encapsulated capacitor maintain its integrity during downstream processing steps such as the thermal excursions associated with solder reflow cycles or overmold baking cycles. Delaminations and/or cracks occurring at any of the various interfaces of the construction or within the layers themselves could undermine the integrity of the embedded capacitor by providing an avenue for moisture penetration into the assembly.
An approach to solve these issues is needed. Various approaches to improve embedded passives have been tried. An example of an encapsulant composition used to reinforce embedded resistors may be found in U.S. Pat. No. 6,860,000, issued to Felten. A crystalline polyimide precursors approach for an organic encapsulant has been described with PCT Patent Application No. PCT/US07/25297. Other polyimide-based encapsulants have been described in U.S. Patent Applications US-2007-0290379-A and US-2007-0291440-A. An approach to organic encapsulants with polynorbornene and polyarylates is U.S. Patent Application US-2007-0154105-A.
Epoxy materials have been described as encapsulants. K. I. Papathomas in U.S. Pat. No. 7,192,997 B2 describes a composition for use in making an encapsulant usable in the encapsulation of a semiconductor chip. Other references using epoxy plus phenolic resins are U.S. Patent Applications US-2007-0244267-A and US-2007-0236859-A.
Polybenzoxazoles may have utility as encapsulants since they generally possess low diffusion coefficients to moisture and gases, high degree of dimensional stability, high toughness, high Tg, low to moderate Cites, low water uptake, and good adhesion. However, in the investigation of the use of polybenzoxazoles in fibers, a problem was identified. With poly-p-phenylenebenzobisoxazole fibers, P. J. Walsh et. al., in the Journal of Applied Polymer Science, Vol. 102, 3819-3829 (2006)], there were issues identified with hydrolytic reactions of weak bases and acids, such as morpholine, pyridine and trimethylphosphate that were used to extract residual phosphoric acid. There was evidence that there were hydrolytic reactions involving the oxazole nitrogen that led to disruption of the oxazole ring structure.
Polybenzoxazoles that are claimed to be soluble in the literature that are based on a solubilizing diamine component, 2,2-bis(3-amino-4-hydroxyphenyl)hexafluoropropane (6F-AP) were described by L. R. Denny et. al. at the 22nd International SAMPE Technical Conference, Nov. 6-8, 1990 and by Houtz et. al., Polymer Preprints, 1994, 35 (1), 437-8. These references describe a search for thermoplastic canopy materials that are tolerant towards aerodynamic heating needed to aid in high speed Air Force applications. The PBOs identified were soluble in non-screen print solvents such as methanesulfonic acid, sulfuric acid, chloroform or THF.
In U.S. Pat. No. 7,064,176B to Halik et. al., soluble polyhydroxyamides that thermally convert to PBOs, were prepared to be used as adhesives to bond chips to electronic packages. Solvents that are infinitely soluble in water were used to spin coat the polymer solutions onto wafers. For screen print applications, most of the solvents indicated are too low boiling and have too much water absorption. The higher boiling solvents indicated, NMP and gamma-butyrolactone, attack the screen emulsion, the squeegee, and are infinitely soluble in water, which makes these solvents unusable for screen-printing.
In WO 2007/034716 A1 to M. Hasegawa et. al., a sulfone-containing PBO is described that has high solubility in solvents, but the highly polar sulfone functionality is anticipated to cause higher than desired moisture absorption into an encapsulated capacitor that is subjected to up to 1000 hours in an 85° C., 85% RH with a 5 volt DC bias.